1. Field of the Invention
The present invention relates generally to an improvement in an EL panel, and particularly concerns a thin film EL panel which is suitable for AC operation. The improved EL panel is configurated very thin and of good viewing capability, and is suitable for graphic and character terminal display apparatus of a computer or the like.
2. Description of the Prior Art
Generally speaking, an EL panel has such a configuration as having on a glass substrate, a transparent electrode such as solid solution oxide of In and Sn, i.e., ITO, a first insulator film, a phosphor film, a second insulator film and a back electrode which is generally of Al, wherein other film than Al film are transparent layers. Accordingly, when viewed through the glass substrate, the laminated thin films being transparent, the light incident through the glass substrate is reflected by the inside surface of the aluminum electrode, thereby making the panel like a mirror.
The case where the incident light is normal to the panel face is described. Among the incident light, about 8% is reflected by the films before the Al electrode; and of the remaining light of 92% of the incident light, for instance, 90% is reflected. As a result, the overall reflectivity of the panel becomes about 91%. In the actual panel, due to absorptions in the thin films and scatterings at interfaces, some percentage of light is further lost, and accordingly the overall reflectivity is about 80-90%. Accordingly, in the conventional EL panel having an average brightness of 100 nit at no incident light shows a considerable brightness-up when viewed in a fairly bright room, for instance at illumination of 400 lux when the incident light and the reflected light are normal to the panel surface, the incident light of about 400 lux is almost reflected thereby undesirable adding the brightness to the black level of about 100 nit, (corresponding to the brightness of the picture elements of black level). As a result of the above-mentioned high reflection, the contrast of the EL panel as display device becomes such low as about 2:1, while a contrast of 5:1 or more is desirable for an EL panel even at the illumination of about 400 lux.
In order to improve the contrast of an EL panel, such proposals are made as arranging a circular polarization filter in front of the EL panel or providing a black insulator film between a phosphor layer and a back electrode. The former measure of using the circular polarization filter has a shortcoming of decreasing the brightness by about 60% and besides it is expensive. In the latter measure, though the brightness is decreased by about 50% the cost is low and practical, but the problem of the latter measure is a difficulty of obtaining black colored thin insulator film. Generally speaking, the black thin insulator film of the EL panel is requested to have a high sheet resistance of above 10.sup.8 .OMEGA./.quadrature., and that light transmittance is preferably as small as substantially 0% for light of visible range, and dielectric constant thereof is preferably as high as possible in order to decrease driving voltage of the EL panel. However, such black thin insulator film has not yet been available so far.